1. Field of the Invention
The present invention relates generally to xerographic printing and, more particularly, to methods and devices for transferring toner in a substantially vertical direction.
2. Background of the Related Art
In modern society, among the most common and useful printing devices are printers that are used in conjunction with computers to print a variety of subject matter, such as text, graphics, and even photographic reproductions. These "computer" printers may be categorized in any number of ways. However, for the purposes of this discussion, these types of printers will be categorized, initially, as monochromatic and color printers. Monochromatic printers use a single color ink or toner, which is a form of powdered imaging material that can be charged and moved with electric fields. Most monochromatic printers are capable of producing gray and black images on a print medium, such as paper, transparencies, etc. Color printers, on the other hand, typically contain several colors of ink or toner, such as cyan, magenta, and yellow, which produce the color images, as well as black, which produces the black and gray images. As described in greater detail below, just as certain monochromatic printers have the ability to produce certain shades of gray, these color images may be produced, to some extent, in different color hues and saturations.
As far as computer printers are concerned, color printers are a relatively recent innovation. Therefore, historically, computer printers have been categorized primarily based upon the type of technology used to deliver the ink onto the paper. Such technological categories of printers have included, for instance, daisy wheel printers, ink jet printers, and laser printers. Arguably the most popular printers in today's market, for both monochromatic and color printers, are ink jet printers and laser printers. Unfortunately, each of these types of printers exhibit certain disadvantages, particularly when used as color printers.
Ink jet printers print directly onto paper. In other words, the ink is not deposited on an intermediate substrate which is then transferred from the intermediate substrate to the paper. Rather, ink jet printers use thermally generated bubbles or piezoelectric drivers to expel or "jet" ink drops onto the print-receiving medium. Advantageously, such printers are relatively inexpensive and operate satisfactorily for a variety of purposes. However, ink jet technology demonstrates very limited gray scale level writing ability at the present time. In other words, ink jet printers can only produce a few shades of gray. To provide these limited gray scale levels, ink jet printers may use diluted and full strength inks, smaller ink drops, or modulated drop sizes. In view of these limitations, ink jet printers are unlikely ever to achieve more than a few gray levels.
Toner jet printers also print directly onto paper. To provide this type of direct printing, toner jet printers typically pass toner through an array of holes that is placed in the print head very near the paper. A ring electrode is placed around each hole to control the toner that passes through each hole. This control is possible because the toner is charged prior to delivering it to the array. Accordingly, activation of an electrode essentially pulls the toner through the activated hole, and an electrode may be placed behind the print medium to pull the toner onto the paper.
The saturation of the toner on the paper may be controlled, to some extent, by the time that the particular electrode is activated. In other words, in a monochromatic printer, the electrode may be activated for a relatively short period to produce a gray image and for a relatively long period to produce a black image. Similarly, in a color printer, the electrode is activated for a relatively short period of time to produce a light colored image and for a relatively long period of time to produce a darker colored image.
Disadvantageously, the holes in the array tend to get plugged with toner, so the arrays need to be cleaned periodically. This maintenance may require the array to be removed from the printer for cleaning or replacement, or the printer may be provided with a self-cleaning mechanism that periodically produces a charge in an attempt to attract the charged toner particles away from the array. In an effort to address these concerns, the holes in the array may be made larger to help alleviate the plugging problem. However, this solution is detrimental because increasing the size of the holes increases pixel size, thereby causing the resolution of the printer to suffer.
Laser printers present another set of advantages and disadvantages. On one hand, laser printers are very reliable, require little maintenance, and are capable of printing at relatively high speeds as compared with ink jet printers. On the other hand, laser printers are more complicated and more expensive than comparable ink jet printers. Furthermore, laser printers are essentially analog devices, and it is difficult to control the analog process tightly enough to get satisfactory color control. Rather, various shades of gray or various color densities are produced by the use of "super pixels," i.e., tight groupings of regular pixels having various different colors and/or densities to produce a given effect when viewed at a distance by the human eye.
In an effort to improve upon existing printers, electrostatic printers using traveling wave toner transport devices, sometimes called digital packet printing devices, are under development. Such devices use microscopic patterns of electrodes that are formed using semiconductor fabrication techniques to control small numbers of toner particles. Because of the precise control of the toner that these devices theoretically make possible, it is thought that these devices could produce print images having a higher resolution and much better gray scale control than existing printers. Furthermore, it is thought that these devices could provide high operating speed at a potentially lower cost. However, known traveling wave toner devices have not attained these theoretical advantages.
As discussed in detail below, the present inventors have discovered a variety of problems with currently known traveling wave toner transport devices, as well as a variety of ways to address such problems and improve traveling wave toner transport technology.